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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>three_way_comparable, <span style="font-size:0.7em; line-height:130%">std::</span>three_way_comparable_with</h1>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../to_chars.html" title="cpp/utility/to chars"><span class="t-lines"><span>to_chars</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../from_chars.html" title="cpp/utility/from chars"><span class="t-lines"><span>from_chars</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../chars_format.html" title="cpp/utility/chars format"><span class="t-lines"><span>chars_format</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 类型运算</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../declval.html" title="cpp/utility/declval"><span class="t-lines"><span>declval</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../as_const.html" title="cpp/utility/as const"><span class="t-lines"><span>as_const</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../launder.html" title="cpp/utility/launder"><span class="t-lines"><span>launder</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code><a href="../../header/compare.html" title="cpp/header/compare">&lt;compare&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span><br>
<p>  concept __PartiallyOrderedWith <span class="sy1">=</span>      <span class="co1">// 仅为阐释</span><br>
    requires<span class="br0">(</span><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> t,<br>
             <span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span> u<span class="br0">)</span> <span class="br0">{</span><br>
      <span class="br0">{</span> t <span class="sy1">&lt;</span>  u <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> std<span class="sy4">::</span><span class="me2">boolean</span><span class="sy4">;</span><br>
      <span class="br0">{</span> t <span class="sy1">&gt;</span>  u <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> std<span class="sy4">::</span><span class="me2">boolean</span><span class="sy4">;</span><br>
      <span class="br0">{</span> t <span class="sy1">&lt;=</span> u <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> std<span class="sy4">::</span><span class="me2">boolean</span><span class="sy4">;</span><br>
      <span class="br0">{</span> t <span class="sy1">&gt;=</span> u <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> std<span class="sy4">::</span><span class="me2">boolean</span><span class="sy4">;</span><br>
      <span class="br0">{</span> u <span class="sy1">&lt;</span>  t <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> std<span class="sy4">::</span><span class="me2">boolean</span><span class="sy4">;</span><br>
      <span class="br0">{</span> u <span class="sy1">&gt;</span>  t <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> std<span class="sy4">::</span><span class="me2">boolean</span><span class="sy4">;</span><br>
      <span class="br0">{</span> u <span class="sy1">&lt;=</span> t <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> std<span class="sy4">::</span><span class="me2">boolean</span><span class="sy4">;</span><br>
      <span class="br0">{</span> u <span class="sy1">&gt;=</span> t <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> std<span class="sy4">::</span><span class="me2">boolean</span><span class="sy4">;</span><br>
</p>
    <span class="br0">}</span><span class="sy4">;</span></span></div></td>
<td> (1) </td>
<td> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span> </td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> Cat<span class="sy1">&gt;</span><br>
<p>  concept __ComparesAs <span class="sy1">=</span>                <span class="co1">// 仅为阐释</span><br>
</p>
    std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1">&lt;</span>std<span class="sy4">::</span><span class="me2">common_comparison_category_t</span><span class="sy1">&lt;</span>T, Cat<span class="sy1">&gt;</span>, Cat<span class="sy1">&gt;</span><span class="sy4">;</span></span></div></td>
<td> (2) </td>
<td> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span> </td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> Cat <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">partial_ordering</span><span class="sy1">&gt;</span><br>
<p>  concept three_way_comparable <span class="sy1">=</span><br>
    __WeaklyEqualityComparableWith<span class="sy1">&lt;</span>T, T<span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
    __PartiallyOrderedWith<span class="sy1">&lt;</span>T, T<span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
    requires<span class="br0">(</span><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> a,<br>
             <span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> b<span class="br0">)</span> <span class="br0">{</span><br>
      <span class="br0">{</span> a <span class="sy1">&lt;=&gt;</span> b <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> __ComparesAs<span class="sy1">&lt;</span>Cat<span class="sy1">&gt;</span><span class="sy4">;</span><br>
</p>
    <span class="br0">}</span><span class="sy4">;</span></span></div></td>
<td> (3) </td>
<td> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span> </td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U, <span class="kw1">class</span> Cat <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">partial_ordering</span><span class="sy1">&gt;</span><br>
<p>  concept three_way_comparable_with <span class="sy1">=</span><br>
    std<span class="sy4">::</span><span class="me2">three_way_comparable</span><span class="sy1">&lt;</span>T, Cat<span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
    std<span class="sy4">::</span><span class="me2">three_way_comparable</span><span class="sy1">&lt;</span>U, Cat<span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
    std<span class="sy4">::</span><span class="me2">common_reference_with</span><span class="sy1">&lt;</span><br>
      <span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span>,<br>
      <span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
    three_way_comparable<span class="sy1">&lt;</span><br>
      std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1">&lt;</span><br>
        <span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span>,<br>
        <span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span>, Cat<span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
    __WeaklyEqualityComparableWith<span class="sy1">&lt;</span>T, U<span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
    __PartiallyOrderedWith<span class="sy1">&lt;</span>T, U<span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span><br>
    requires<span class="br0">(</span><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> t,<br>
             <span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span> u<span class="br0">)</span> <span class="br0">{</span><br>
      <span class="br0">{</span> t <span class="sy1">&lt;=&gt;</span> u <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> __ComparesAs<span class="sy1">&lt;</span>Cat<span class="sy1">&gt;</span><span class="sy4">;</span><br>
      <span class="br0">{</span> u <span class="sy1">&lt;=&gt;</span> t <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">&gt;</span> __ComparesAs<span class="sy1">&lt;</span>Cat<span class="sy1">&gt;</span><span class="sy4">;</span><br>
</p>
    <span class="br0">}</span><span class="sy4">;</span></span></div></td>
<td> (4) </td>
<td> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<div class="t-li1"><span class="t-li">1)</span> 仅用于阐释的概念 <code>__PartiallyOrderedWith&lt;T, U&gt;</code> 指定 <code>T</code> 类型对象与 <code>U</code> 类型对象能（以任何顺序）用 <code>&lt;</code> 、 <code>&gt;</code> 、 <code>&lt;=</code> 及 <code>&gt;=</code> 在一个偏序中彼此比较，且比较结果一致。</div>
<div class="t-li1"><span class="t-li"></span> 更正式而言， <code>T</code> 与 <code>U</code> 实现 <code>__PartiallyOrderedWith&lt;T, U&gt;</code> ，仅若给定
<ul><li> <code>t</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span></span></span> 类型左值，与
</li><li> <code>u</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span></span></span> 类型左值
</li></ul>
<p>下列为真：
</p>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">&lt;</span> u</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">&lt;=</span> u</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">&gt;</span> u</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">&gt;=</span> u</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">u <span class="sy1">&lt;</span> t</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">u <span class="sy1">&lt;=</span> t</span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp">u <span class="sy1">&gt;</span> t</span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp">u <span class="sy1">&gt;=</span> t</span></span> 拥有相同定义域；
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">&lt;</span> u<span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>u <span class="sy1">&gt;</span> t<span class="br0">)</span></span></span> ；
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>u <span class="sy1">&lt;</span> t<span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">&gt;</span> u<span class="br0">)</span></span></span> ；
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">&lt;=</span> u<span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>u <span class="sy1">&gt;=</span> t<span class="br0">)</span></span></span> ；且
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>u <span class="sy1">&lt;=</span> t<span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">&gt;=</span> u<span class="br0">)</span></span></span> 。</li></ul></div>

<div class="t-li1"><span class="t-li">3)</span> 概念 <code>three_way_comparable&lt;T, Cat&gt;</code> 指定三路比较运算符 <code>&lt;=&gt;</code> 在 <code>T</code> 上生成与 <code>Cat</code> 所蕴含的比较类别一致的结果。
<p><code>T</code> 与 <code>Cat</code> 实现 <code>three_way_comparable&lt;T, Cat&gt;</code> ，仅若给定 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span></span></span> 类型左值 <code>a</code> 与 <code>b</code> ，下列为真：
</p>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy1">&lt;=&gt;</span> b <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy1">&lt;=&gt;</span> b <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>a <span class="sy3">!</span><span class="sy1">=</span> b<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="br0">(</span>a <span class="sy1">&lt;=&gt;</span> b<span class="br0">)</span> <span class="sy1">&lt;=&gt;</span> <span class="nu0">0</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="nu0">0</span> <span class="sy1">&lt;=&gt;</span> <span class="br0">(</span>b <span class="sy1">&lt;=&gt;</span> a<span class="br0">)</span><span class="br0">)</span></span></span> 相等，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">&gt;</span> b<span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>b <span class="sy1">&lt;</span> a<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">&gt;=</span> b<span class="br0">)</span> <span class="sy1">==</span> <span class="sy3">!</span><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">&lt;</span> b<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">&lt;=</span> b<span class="br0">)</span> <span class="sy1">==</span> <span class="sy3">!</span><span class="kw4">bool</span><span class="br0">(</span>b <span class="sy1">&lt;</span> a<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy1">&lt;=&gt;</span> b <span class="sy1">&lt;</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">&lt;</span> b<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy1">&lt;=&gt;</span> b <span class="sy1">&gt;</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">&gt;</span> b<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy1">&lt;=&gt;</span> b <span class="sy1">&lt;=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">&lt;=</span> b<span class="br0">)</span></span></span> ，且
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a <span class="sy1">&lt;=&gt;</span> b <span class="sy1">&gt;=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">&gt;=</span> b<span class="br0">)</span></span></span> ；而
</li><li> 若 <code>Cat</code> 可转换为 <span class="t-lc">std::strong_ordering</span> ，则 <code>T</code> 实现 <a href="../../concepts/totally_ordered.html" title="cpp/concepts/totally ordered"><tt>totally_ordered</tt></a> 。</li></ul></div>

<div class="t-li1"><span class="t-li">4)</span> 概念 <code>three_way_comparable_with&lt;T, U, Cat&gt;</code> 指定三路比较运算符 <code>&lt;=&gt;</code> 在（可能混合的） <code>T</code> 与 <code>U</code> 上操作数上生成与 <code>Cat</code> 所蕴含的比较类别一致的结果。比较混合的操作数生成的结果等价于比较转换到其功用类型的操作数。
<p>正式而言， <code>T</code> 、 <code>U</code> 及 <code>Cat</code> 实现 <code>three_way_comparable_with&lt;T, U, Cat&gt;</code> 仅若给定
</p>
<ul><li> <code>t</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span></span></span> 类型左值，与
</li><li> <code>u</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span></span></span> 类型左值
</li></ul>
<p>令 <code>C</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">common_reference_t</span><span class="sy1">&lt;</span><span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span>, <span class="kw4">const</span> <a href="../../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="sy1">&gt;</span></span></span> ，下列为真：
</p>
<ul><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">t <span class="sy1">&lt;=&gt;</span> u</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">u <span class="sy1">&lt;=&gt;</span> t</span></span> 拥有相同定义域；
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="br0">(</span>t <span class="sy1">&lt;=&gt;</span> u<span class="br0">)</span> <span class="sy1">&lt;=&gt;</span> <span class="nu0">0</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="nu0">0</span> <span class="sy1">&lt;=&gt;</span> <span class="br0">(</span>u <span class="sy1">&lt;=&gt;</span> t<span class="br0">)</span><span class="br0">)</span></span></span> 相等；
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>t <span class="sy1">&lt;=&gt;</span> u <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">==</span> u<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>t <span class="sy1">&lt;=&gt;</span> u <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy3">!</span><span class="sy1">=</span> u<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp">Cat<span class="br0">(</span>t <span class="sy1">&lt;=&gt;</span> u<span class="br0">)</span> <span class="sy1">==</span> Cat<span class="br0">(</span>C<span class="br0">(</span>t<span class="br0">)</span> <span class="sy1">&lt;=&gt;</span> C<span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>t <span class="sy1">&lt;=&gt;</span> u <span class="sy1">&lt;</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">&lt;</span> u<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>t <span class="sy1">&lt;=&gt;</span> u <span class="sy1">&gt;</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">&gt;</span> u<span class="br0">)</span></span></span> ，
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>t <span class="sy1">&lt;=&gt;</span> u <span class="sy1">&lt;=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">&lt;=</span> u<span class="br0">)</span></span></span> ，且
</li><li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>t <span class="sy1">&lt;=&gt;</span> u <span class="sy1">&gt;=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw4">bool</span><span class="br0">(</span>t <span class="sy1">&gt;=</span> u<span class="br0">)</span></span></span> ；而
</li><li> 若 <code>Cat</code> 可转换为 <span class="t-lc">std::strong_ordering</span> ，则 <code>T</code> 与 <code>U</code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">totally_ordered_with</span><span class="sy1">&lt;</span>T, U<span class="sy1">&gt;</span></span></span> 。</li></ul></div>

<p>其中 <code>__WeaklyEqualityComparableWith</code> 是仅用于阐释的概念，亦为 <a href="../../concepts/equality_comparable.html" title="cpp/concepts/equality comparable"><tt>equality_comparable</tt></a> 所用。
</p>
<h3><span class="mw-headline" id=".E7.9B.B8.E7.AD.89.E6.80.A7.E4.BF.9D.E6.8C.81">相等性保持</span></h3>
<p>若表达式对给定的相等输入产生相等输出，则它<i>保持相等性</i>。
</p>
<ul><li> 表达式的输入由其运算数组成。
</li><li> 表达式的输出由其结果和表达式所修改的所有运算数（若存在）组成。
</li></ul>
<p>进一步要求每个要求保持相等性的表达式都<i>稳定</i>：这种表达式带相同输入对象的二次求值必须拥有相等的输出，而无任何对这些输入对象的显式中间修改。
</p><p>除非另外提醒，每个用于 <i>requires 表达式</i>中的表达式都要求保持相等性且稳定，而表达式的求值必须只修改其非常运算数。必须不修改常运算数。
</p>
<h3><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F.E5.8F.98.E7.A7.8D">隐式表达式变种</span></h3>
<p>使用不修改某 const 左值运算数的表达式的 <i>requires 表达式</i>亦隐式要求该表达式的额外变种对给定运算数接受非 const 左值或（可为 const 的）右值，除非以有区别的语义显式要求这种表达式变种。这些<i>隐式表达式变种</i>必须符合与声明的表达式的相同的语义。不指定实现以何种程度校验变种的语法。
</p>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../../concepts/equality_comparable.html" title="cpp/concepts/equality comparable"> <span class="t-lines"><span>equality_comparable</span><span>equality_comparable_with</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div>
</td>
<td>  指定运算符 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> 为等价关系 <br> <span class="t-mark">(概念)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../../concepts/totally_ordered.html" title="cpp/concepts/totally ordered"> <span class="t-lines"><span>totally_ordered</span><span>totally_ordered_with</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div>
</td>
<td>  指定比较运算符在该类型上产生全序 <br> <span class="t-mark">(概念)</span> </td></tr>
</table>

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